Because there event pokemon, Mew is Event only in RBY and Celibi is Event only In GSC( Crystal only in Japan) to get Mew use the Mew glitch in RBY and trade it. Or you could use a Gameshark to hack him in. You need to hack Celebi to get one.
"I reject your reality and substitute my own!" - Adam Savage Pearl FC: 0559 3455 4190. Name Conker
There's a video on YouTube that shows a way you can supposedly get Celebi through a glitch. I haven't tried it so I can't vouch whether it works or not. Looks like it has the potential to screw up your data if you do it wrong though.
There is a way to obtain Mew with a glitch on Red, Blue, and Yellow versions, that I have seen actually work with my own eyes. You can trade them to Gold after that.
Okay, I tried the Celebi glitch as shown in the above video I linked to, and I can't get it to work. I tried it numerous times with no success, leading me to believe that the video is fake, but I can't be sure about that.